Iris Borer

Iris Borer

Part of Ep. 704 The Summer Garden

Join UW-Extension Entomologist Phil Pellitteri as he delves into the problems of the iris borer.

Premiere date: Aug 28, 1999

TRANSCRIPT+
Wisc Gardener Transcript: 

Shelley:
Mid-to-late summer is a great time of year to be digging up and dividing your iris. It helps with crowding and it also helps prevent other problems in the garden. We brought these roots indoors because it's a little too wet outside to be doing this. This is a healthy iris root. Hopefully, this is what you find when you dig them up and divide them. Sometimes, however, you run into problems. With me is UW-Extension Entomologist, Phil Pellitteri. Phil, this is what I'd like see when I'm digging my iris up. What might I see?

Phil:
Unfortunately, iris has one big problem. It's an insect problem called iris borer. In about July or August, about this time of the year, if you were to dig it up and had iris borer, what you would see is a big pinkish or yellowish or brownish caterpillar feeding extensively on the rhizomes. And you can see all this chewed area in here where they fed.

Shelley:
That is not a pleasant looking insect, either.

Phil:
No. And unfortunately, aside from this physical chewing, it opens the plant up to a bacterial soft rot, which is a disease. We have some soft rot over here. And all you see is a yellowing of the plants. Notice how this is just-- there's nothing there anymore.

Shelley:
It's just mush.

Phil:
It's mush. And you'll notice it has kind of a distinctive foul odor.

Shelley:
Yeah, I'm noticing that.

Phil:
And so, soft rot is not something you want. What's happened is the iris borer-- it's almost like a wound. It opened up the rhizome and then the soft rot moves in.

Shelley:
This, I'm afraid to touch this, it almost looks like wet-- the outer coating of this healthy rhizome.

Phil:
That's all that's left. When it really gets going, it just becomes mush. And as I said, you might find a caterpillar, but you might just find the soft rot. What started the soft rot, unfortunately, though, was a caterpillar.

Shelley:
The iris borer. Okay, what do we do about it? I assume when I'm digging these up, I don't want to plant something like that back into the ground.

Phil:
No. Usually, when you're transplanting these plants if you see this, you can cut that bad section off, get rid of the caterpillars you find and then you can just plant the good, sound, healthy stuff like you have here on good plant.

Shelley:
Just put that back in the ground. Will that take care of the problem?

Phil:
Unfortunately not. The iris borer is a night flying moth and it looks much like many of the cut worm moths. It belongs to the cut worm moth family. What's interesting about this little critter is it was first discovered around the northern suburbs of Chicago in the 1890s on daylilies.

Shelley:
Daylilies?

Phil:
We don't see a lot of damage of daylilies because iris by far is it's favorite thing to attack. Again, if you've grown irises very long, unfortunately, you've probably dealt with the iris borer. Now, the moth is out in August and September. She lays her eggs around the base of the plant and the areas around it. That's how the insect gets through the winter.

Shelley:
So, to maybe prevent some of the problem when we're cleaning up in fall like after the first frost, get rid of the plant debris and the leaves lying around the iris.

Phil:
It does make a big difference. Unfortunately, you don't get all the eggs. Long in the spring, when we're getting about three inches of growth, the caterpillars are hatching. They start to migrate onto the plant. And as the name borer tells you, what they do is they find a spot and they start to crawl in. You often see a discolored area where they start. And over time, they will move down the plant toward the rhizome. Sometimes, people get very poor blooms because they feed on the developing flowers before they emerge. So, an iris borer causes that kind of damage also. But eventually, when you get into late June and July, then they're down in the rhizome causing problems with the root.

Shelley:
So, then we're back to full circle and we've got these mushy roots. And again, it's too late to do anything at this point other than get rid of the rotting part.

Phil:
Right. So, we look at what kind of control options we have. Up until a couple of years ago, we really only had one. There is an insecticide called dimethoate or Cygon, which is one of the more toxic insecticides. We usuall don't like to recommend it. But unfortunately, it's the only thing that we found worked against the iris borer. The way it's applied is when you get about three inches of growth in the spring, you'll apply it the first time. And then about two weeks later, it's good to use a follow-up spray of this. So, up until about two or three years ago, that was the only suggestion we could make to people other than cleaning the plant debris-- to use this dimethoate insecticide.

Shelley:
A lot more gardeners have increasing concerns about using toxic chemicals. Is there another alternative now?

Phil:
There is. It's something you have to do a little homework on. It's use of parasitic nematodes.

Shelley:
What are nematodes?

Phil:
A Nematode is a microscopic worm. And there are some nemitodes that are plant feeders that are plant pests, but there are some that are insect feeders. And that's what you can commercially buy now. We've brought a couple types here that we bought through a catalog.

Shelley:
This isn't something that I'm going to pick up at the local hardware store.

Phil:
Not yet. Maybe some day. If people ask for it enough, garden stores might carry it. But right now, you have to order this. They'll ship it to you in various methods. In this case, we've got five million nematodes that have been impregnated on this sponge. Another form of it is this has 50 million nematodes that are impregnated on an almost clay-like residue. And a third form that you will see at times is a sawdust-like material that is mixed in. This is put on almost like a salt shaker. You can shake it on. The way these things are used the best-- take this sponge and what you do is get a bucket of water.

Shelley:
So, all of these basically go into water, whatever form you buy it in.

Phil:
In order to work the best, they need to be mixed in water. We just put the sponge in there and let it sit for a couple minutes so the nematodes have a chance to swim out into the water. Then we take this out to the plants and drench it around the base of the plants. And what the namatodes will do is work down into the soil into where they find these iris borers and kill them.

Shelley:
So, basically just pour this on the iris plants, pour it onto the soil around them and that's it?

Phil:
Right. What's neat about this is that we can only use the sprays for about two weeks. The nematodes are effective for about four or five weeks. You can even go well into June and early July and still get some success with using the nematodes.

Shelley:
Are there any other toxic after effects of this?

Phil:
No. If you're an insect, this is a problem, but anything else out there as far as wildlife is not affected by the nematodes.

Shelley:
This is a great choice then. But that's why we should be looking for this problem in late summer. It may take us a little bit of time to find this.

Phil:
This is going to have to be mailed to you. The nice thing about nematodes is you can keep them in the refrigerator for a couple weeks. But you basically have to have them here when you're going to use them. You have to be prepared to find where you can get them and then make sure they're shipped at the right time.

Shelley:
Okay, great. This is a safe alternative. Thanks, Phil. So, now that you've got these dug up and you're ready to plant them back into the ground, cut the leaves off to about here, maybe about three inches and plant these very shallowly, only about two inches in the soil. Then you're set for next spring.

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